Vacuum coating apparatus

ABSTRACT

Disclosed is a method and apparatus for coating, impregnating or otherwise surface treating boards, strips, sheets, planks and similarly shaped elongated workpieces suitably of lumber, but also of metal, boxboard and so on. A series of workpieces are fed into a coating chamber maintained at subambient pressure through openings lined with flexible wipers which define apertures generally n conformity with cross-sectional periphery of the workpiece being coated. One or more of the surfaces of the workpiece are flushed with a liquid coating composition within the chamber by flowing an excess of what remains on the workpiece, the excess coating draining by gravity to a reservoir for recycle. Upon leaving the chamber, the flexible wiper on the outlet aperture acts as a squeegee to assist in sealing the outlet while the workpiece is passing through and to wipe the coating free of air bubbles and solid contaminants.

United States Patent 91 Story YACUUM COATING APPARATUS [75] Inventor: Laurence A. Story, Burnaby, Canada [73] Assignee: SCM (Canada) Ltd., Toronto,

Ontario, Canada [22] Filed: Apr. 14, 1971 [21] Appl. No.: 133,810

[52] US. Cl 118/50, 118/109, 118/125, 118/316, 118/326, 118/603 [51] Int. Cl...... 1305c 5/00, 1305c 11/04, B05c 11/16 [58] Field of Search l18/DIG. 9, 316, 118/326, 602, 603, 109, 125, 50, DIG. l1

[451 July 17, 1973 Primary Examiner-John P. McIntosh Attorney-Merton H. Douthitt 57 ABSTRACT Disclosed is a method and apparatus for coating, impregnating or otherwise surface treating boards, strips, sheets, planks and similarly shaped elongated workpieces suitably of lumber, but also of metal, boxboard and so on. A series of workpieces are fed into a coating chamber maintained at subambient pressure through openings lined with flexible wipers which define apertures generally 11 conformity with cross-sectional periphery of the workpiece being coated. One or more of the surfaces of the workpiece are flushed with a liquid coating composition within the chamber by flowing an excess of what remains on the workpiece, the excess coating draining by gravity to a reservoir for recycle. Upon leaving the chamber, the flexible wiper on the outlet aperture acts as a squeegee to assist in sealing the outlet while the workpiece is passing throughand to wipe the coating free of air bubbles and solid contaminants.

5 Claims, 9 Drawing Figures ruimtm m I saw a ur s EXHAUfiTEE PUMP v Pmmmm v 1 m SHEET 5 BF 5 Fig. 8

VACUUM COATING APPARATUS This invention relates to an and apparatus for applying surface coatings to a series of articles or workpieces of similar cross-sectional configuration. More particularly, this invention concerns an and apparatusfor applying liquid coating to a series of elongated bodies or strip material in the form of bars, rods, boards, sheets, planks and the like. In a particularly preferred embodiment, lumber in the form of boards is coated in rapid succession in assembly-line fashion in conjunction with a sawmill operation.

In applying decorative and functional coatings such as paints, varnishes, enamels, stains, lacquers and water repellents to several workpieces of similar dimensional configuration, the usual time-consuming method of hand brushing, spray painting and dip coating are often commercially unattractive because of inefliciency of application and high labor cost. These high costs are particularly burdensome when the coated workpiece is to be sold at a relatively low cost, such as in the case of lumber.

Freshly cut and trimmed lumber often requires tern porary and/or permanent protective coatings to prevent staining and discoloration upon shipping and storage. Furthermore, under conditions of outdoor exposure, coatings are often required to insure dimensional stability and to repel termites and insects. In some cases, lumber is actually coated with a permanent decorative and/or protective coating at the mill site prior to shipping.

Accordingly, a substantial advance in the art would be realized by providing an inexpensive and efficient apparatus for applying decorative or functional liquid coating to a series of similarly shaped workpieces such as cut and trimmed lumber.

U. S. Pat. No. 3,084,662 proposes one type of apparatus for such coating operations. This patent discloses a coating tank containing a substantial volume of liquid coating material. The tank is connected to a vacuum system at a point above the liquid level. The coating tank has an inlet and outlet for the boards positioned below the liquid level in the tank. The pressure is reduced in the coating tank while boards are continuously passed through the coating tank in submerged fashion below the liquid level. The boards become coated under the influence of a hydrostatic head of paint, and excess coating material is removed from the boards as the boards pass through a gasket positioned in the outlet. A partial vacuum in the coatingtank is said to prevent the liquid from flowing out of the tank through the openings for the boards.

While this prior art design has several advantages, its substantial vapor and liquid losses can be costly and detract from the efficiency of the operation. Additionally, a certain degree of operator attention is required to maintain the rather delicate balance between the vacuum system and the lumber size, line speed and liquid coating characteristics to minimize coating losses at the interface between the board and the gasket. Furthermore, the air drawn into the chamber at the boardgasket interface must bubble through a substantial volume of liquid in the coating chamber, and in so doing acts to vaporize or entrain liquid coating to the vacuum system. This often results in foaming in aqueous-based coatings and substantial solvent loss as well as substantial thickening in the organic solvent-based coatings which detracts from the overall efficiency of the operation. Moreover, it is not possible to coat less than all sides of the workpiece by this prior design because the coating operation is essentially an immersion technique and all exposed surfaces of the workpiece must be coated. This immersion technique can also limit the coating speed since the workpiece must pass through a substantial liquid volume which has a decelerating effect with respect to the speed of the workpiece entering the coating tank.

U. S. Pat. No. 1,263,858 discloses apparatus for applying coating comprising a hollow fitting with apertures therein, through which the article to be coated passes longitudinally with some clearance. Paint is pumped into the fitting to flood the article, and excess liquid coating is withdrawn from the fitting by a suction pump which is adapted to withdraw fluid from the fitting at a faster rate than that at which the coating material is being supplied to the fitting. The suction pump thus draws air into the fitting through the clearance be tween the aperture and the article. This system removes air and excess paint from the fitting in the same stream, and thus the paint tends to be aerated and foamy. This is particularly troublesome in the case of aqueous latex or emulsion coatings as foamy coatings provide poor coating upon recirculation to the coating chamber. Moreover, the clearance between the apertures of the hollow fitting and the article being coated allows air to rush into the fitting to further aerate the paint. Furthermore, lumber in a sawmill is processed at line speeds of 300 feet per minute to as high as 1,500 feet per minute. At these very high line speeds, sawdust, woodchips and other wood fragments are knocked from the boards by contact with the processing equipment. These wood fragments tend to accumulate in the liquid coating and deposite as solid contaminants in the coating on the article. The apparatus of U. S. Pat. No. 1,263,858 makes no provision for removing such solid contaminants either from the liquid coating or the coated workpiece.

In view of the above, the objects of the present invention are to provide an improved coating apparatus which particularly adapts to the needs of the lumber industry.

In accordance with the primary features of the invention, there is provided an and apparatus for continuously coating a series of elongated workpieces with a decorative or functional coating by utilizing a vacuumassisted squeegee mechanism. As used herein, the term squeegee (sometimes referred to in the art as squib gee) refers to the wiping action achieved by drawing flexible blade-like, brush-like or roller-like wiping element over a coated surface. Generally, the vacuum squeegee coating apparatus is a partially evacuated tank equipped with a workpiece inlet and outlet having a flexible wiper on the workpiece outlet which serves to wipe the coating to remove air bubbles and solid contaminants. The outlet wiper is forced against the workpiece by the ambient air pressure and acts as a seal to prevent liquid from leaving the chamber and air from entering the chamber. The liquid coating is applied to the boards by flushing or flooding with a stream of coating liquid from a suitable coating dispenser positioned within the tank. Excess liquid coating is wiped from the workpiece by the wiper on the outlet and drains by gravity to a sump or reservoir in the base of the tank. A separator removes gross solid contaminants from the liquid coating, and the liquid coating is recirculated to the coating dispenser for reuse.

For efficiency and economy, workpiece inlets and outlets of the tank are usually both provided with wip ers to prevent the ingress of too much air through the inlet opening. A large volume of air would require a very high capacity exhauster. These wipers are flexible, and preferably resilient, blades, sleeves, gaskets, brushes, bushings or rollers closely approximating the cross-sectional periphery of the workpiece, but generally slightly smaller to restrict inlet of surrounding air. The reduced pressure in the tank causes the wiper on the workpiece inlet to tend to open to admit the workpiece while the partial vacuum in the tank prevents the coating from flowing out of the workpiece inlet. Conversely, the partial vacuum in the tank causes the exit wiper to be drawn tightly against the workpiece thus forming a liquid seal. This seal causes the coating material to spread out while being worked into the workpiece. 7

The above and other features and advantages of the present invention will become more apparent from considering the detailed description and drawings wherein:

FIG. 1 is a process schematic illustrating the steps in the preparation and coating of boards;

FIG. 2 is a front elevational view of an apparatus embodying the invention;

FIG. 3 is a back elevational view in partial section of the apparatus illustrated in FIG. 2;

FIG. 4 is a side elevational view in partial section of the apparatus of FIGS. 2 and 3;

FIG. 4a is a view of a removable access door for use as shown in FIG. 4;

FIG. 5 is a detailed, exploded view illustrating constructional features of a wiper assembly suitable for use with the apparatus of FIG. 2;

FIG. 6 is a side view of one of the wiper elements illustrated in FIG. 5;

FIG. 7 is another form of wiper design suitable for use with the apparatus of FIG. 2;

FIG. 8 is an exploded view of the wiper of FIG. 7.

It can be readily appreciated that there is a need for coating trimmed boards at the lumber mill to prevent discoloration and preserve stability upon storage and transit. Many types of liquid coating compositions are used to achieve these and other desired coating properties, and the present invention is not limited to the application of any particular type of liquid coating. Accordingly, the term liquid coating composition as used herein includes all decorative or functional liquid coating compositions such as liquid paints, lacquers, varnishes,.stains, preservatives and insect repellents including solutions and dispersions of resinous materials or crosslinkable mixtures in organic solvents; and aqueous solutions, latex emulsions, dispersions or suspensions.

Referring now to the drawings, FIG. 1 illustrates schematically the process sequence for preparing lumber for marketing. In that boards trimmed from lumber are one of the most important embodiments of the present invention, the present invention will be described in terms of lumber, although it will be apparent to those skilled in the art that other workpieces can be similarly coated. Lumber is passed through a planer where the boards are planed to the desired nominal board sizes such as 2 X 2, 2 X 12, 4 X 4 inches and so on. The boards are then conveyed through the coater wherein the desired coating is applied. After leaving the coater, the coated boards are conveyed through a drier if a drying step is desired or required by the nature of the liquid coating. The drier is often equipped with heating elements in the form of steam coils, electrical heaters, indirect heaters, microwave, ultraviolet or electron beam devices to dry and/or force cure the coating. After leaving the drier, the finished boards are ready for the storage or shipment. In this processing sequence, the boards are conveyed by conventional materials handling equipment such as drive rolls, belt conveyors or chain conveyers.

Referring now to FIGS. 2 through 8, 10 generally indicates an airtight chamber or tank of mild steel or other conventional construction material chemically compatible with the liquid coating composition being used having a lid 11 for inspection and cleaning. Posi tioned on a side wall of tank 10 is inlet 12 for the introduction of boards 21 into tank 10. The inlet 12 adapted to receive a series of boards being fed to the tank 10 in end-to-end fashion but not necessarily in abutting relationship. An outlet 13 is similarly positioned on the side wall of the tank opposite from inlet 12 so as to be substantially in horizontal alignment with inlet 12 to define a path of board travel transversing tank 10.

The base of tank 10 is mounted on support plate 34. Support plate 34 is equipped with wheels 32 which are adapted to fit into track 33 for moving the apparatus in and out of the lumber processing line.

Mounted adjacent tank 10 on support plate 34 is recirculation pump 42. Pump 42 can be any type of pump such as a positive displacement pump, centrifugal pump, air-lift or diaphragm pump, although a diaphragm pump is preferred in conjunction with aqueous liquid coating compositions to minimize shear stress on the coating composition to prevent foaming while preserving coating stability. Pump 42 is connected to the base of tank 10 through conduit 43 which enters tank 10 through fitting 44. Pump 42 discharges into conduit 62, which is connected to inlet fitting 29 of coating dispenser 14. The liquid coating which is not dispensed in dispenser 14 returns to the base of tank 10 through dispenser outlet fitting 30, conduit 45 and base inlet 46. Pump 42 is vented to tank 10 through vent line 49 to prevent vapor locking in the pump.

Above base inlet 46 on tank 10 is positioned liquid level control probe 47 in the form of a float. Liquid control probe 47 activates control cell 47a which opens control valve 48 when the liquid level in the base of tank 10 falls below the set point level. Control valve 48 opens to admit make-up liquid coating composition to the suction of pump 42 until the set point liquid level is restored in the base of tank 10. The liquid level control probe 47 can also be in the form of a photoelectric or conductivity cell when these properties in the liquid coating composition are more easily measured. The base of tank 10 thus forms a reservoir for liquid coating from which recirculation pump 42- takes suction.

Referring specifically to FIG. 4, it is seen that outlet 13 comprises an opening in the wall of the tank which is partially restricted by flexible wiper 18 which is secured to tank 10 by mounting bracket 19. Inlet 12 is similarly equipped with a flexible wiper 15 which is secured to tank 10 by mounting bracket 16. FIG. 4 illustrates boards 21 such as 2 X 10 inches passing through tank 10 in end-to-end fashion. The boards 21 are driven by drive roll 23 through guide rolls 22 at line speeds in the neighborhood of 500 to 600 feet per minute.

Referring specifically to FIGS. 4, 5 and 6, the flexible outlet wiper 18 (and inlet wiper is seen to comprise an assembly of four brush-like elements 35, 36, 37 and 38 which are slidably positionable within mounting bracket 19 (and mounting bracket 16) to accommodate any size of board. For instance, elements 38 and 36 can be moved toward each other for narrow boards and away from each other for wider boards. Because the boards 21 being coated have a rectangular crosssectional periphery, there are four brush-like elements used in the assembly. If the boards were triangular in cross-sectional periphery, then three brush-like elements would be used; if the boards were pentagonal, the five brush-like elements would be used and so on. Each brush-like element 35 through 38 includes a shank, 35a through 38a supporting a plurality of flexible, resilient bristles 35b through 38b. These bristles can be of natural or syntheitc materials, although synthetic plastic bristles such as nylon, polyethylene and polypropylene having a length of l to 3 inches and a diameter of 5 to 50 mils, and preferably 10 to are particularly effective for the present purposes. The brislte can be flagged (wide at the exposed tips). A side view of brush-like element 38 is shown in FIG. 6.

In FIGS. 7 and 8, there is illustrated another type of wiper formed by securing together a plurality of flexible, resilient bristles 39. The bristles are of the design and characteristics described in relationship to FIGS. 5 and 6. The bristles are permanently secured together by an adherent agent such as thermosetting or thermoplastic adhesive to form bristle bundles 40. The bristle bundles 40 at the comer are overlapped and interwoven to form a permanent wiper 27 conforming to the contour of the board being coated. The fibrousbundles are equipped with retaining plates 41 which are suitably of metal or any other rigid material capable of withstanding the force of ingress and egress exerted by the boards. The wiper 27 can be secured to outlet 13 and inlet 12 by means of mounting brackets 19 and mounting bracket 16, respectively.

Positioned inside of tank 10 as shown in FIG. 3 in surrounding relationship to the path through tank 10 defmedby inlet 12 and outlet 13 is coating dispenser 14 in the form of a header pipe having perforations 20. The perforations 20 face the path of travel of the boards from inlet 12 to outlet 13 so that any liquid coating flowing through the perforations 20 will flow on the boards. The dispenser can also be in the form of a slitted pipe or array of spray nozzles or other spray or flow coating device. Dispenser 14 can also be designated to direct liquid coating preferentially to predetermined surface areas of the board when this is desired or required. Liquid coating composition is supplied to dispenser l4, and coating which does not flow through perforations 20 returns for recirculation through outlet fitting 30. Dispenser 14 is equipped with a clean-out plug 31 which can be removed for cleaning out the dispenser 14. Inside of tank 10 above dispenser 14 is mounted entrainment baffle plate 58 for disengaging liquid from gases having a plurality of apertures 59 for passage of gases and vapors. Connected to tank 10 above entrainment baffle plate 58 is exhauster conduit 63 which connects to exhauster 24 through valve 60. Exhauster 24 is a mechanical vacuum pump such as those employed in vacuum cleaners although air jet or stream jet eductors can also be used as the exhauster. Thus, the internal volume of tank 10 above the liquid reservoir defines a liquid-gas disengagementzone from which gas is exhausted through exhauster conduit 63 by means of exhauster 24. 1

Below dispenser 14 within tank 10 is positioned splash plate 25 which receives the excess liquid coating flowing from boards2l. The excess liquid coating flows along splash plate 25 and strikes baffle plate 26 which directs the liquid coating to a first filter tray 64 which has a plurality of large perforations 28 on the order of 1% inch in diameter which serves to remove large particles of solid contaminants. The first filter tray 64 is slidably mounted within tank 10 for easy removal for cleaning on guide members 50 which are secured to the sides of tank 10.

Directly beneath the first filter tray 64 is the second filter tray 51 which is similar to the first filter tray except that the perforations 51a are on the order of Y4 inch. Second filter tray is similarly slidably mounted on tank 10 on guide members 52 and receivesthedrainage through first filter tray 64. Second filter tray 51 serves to further remove solidcontaminants from the liquid coating.

Directlybeneath second filter tray 51 isa third filter tray 53 which has perforations 53a on the order of It; inch in diameter. Third filter tray 53 is similarly slidably mounted in tank 10 on guide members 54. Third filter tray serves to further remove solid contaminants from the liquid coating. Alternatively,.the perforations in the filter trays can be inthe form of slots ratherthan apertures aslong as thefilter trays are adapted to remove gross solid contaminants. The smallest or narrowest perforations in the filter should be approximately the size of the perforations 20 in the coating dispenser 14 so thatsolid contaminants that would have a tendency to plug or restrict the dispensing perforations 20 will be removed. Very small wood fragments such as sawdust and wood flour can be circulated through the coating apparatus without detracting from the coating efficiency or performance. In fact, it is not desirable to have a filter so fine that it will remove sawdust because sawdust will plug the filter resulting in frequent shutdowns for cleaning. It is therefore desirable that the filter only removes such large solid contaminants that would plug or restrict the dispenser perforations 20. The filter system is one form of separator element suitable forremoval of solids.()ther suitable separatorelements include floatation separator where floating solid contaminants are physically removed or skimmed off from the surface of liquid reservoir.

On the side wall of tank 10 directly adjacent filter trays 6.4, 51 and 53 are guide brackets 57 for securing access door 55. Access door 55 is equipped with handles 56 and illustrated in FIG. 4a. Access door 55 is designated for insertion into guide brackets 57 for easy access to filter trays 64, 51 and 53. Access door 55 is secured in position on tank 10 by clamps 61.

In operation, boards 21 passing through inlet wiper 15 onto tank 10 are flushed with liquid coating composition from dispenser 14. In that the boards are flushed with coating, coating easily flows to fill all the cracks, voids and other surface imperfections and irregularities in the boards. The flushed coated boards then pass through outlet 13 and are guided by rollers 22 by drive roll 23 to the drier or storage as indicated in liquid in FIG. 1.

While the boards 21 are passing through the tank, the exhauster 24 reduces the pressure in tank 10 which induces a tight seal between the wiper 18 due to the subambient pressure existing within the coating tank 10.

The ambient air pressure acts on the external parts of wiper 18 to effect a squeegee action on the coated board to wipe away bubbles, blisters or other surface imperfections in the applied coating. Wiper 18 also serves to wipe away any solid contaminants such as sawdust or wood fragments that may deposit with the coating. A pressure differential of about 10 to 30 inches of water below ambient air pressure is adequate for providing efficient wiping action with coating materials having a viscosity of water, although greater or lesser pressure differential between the tank and the surrounding atmosphere can be used (generally, greater pressure differentials will be used with more viscous coating materials). The greater the pressure differential, the greater will be the force on the wiper 18 against the boards. After leaving outlet 13, the boards 21 are firctionally engaged by rollers 22 and 23 which remove the boards from tank 10.

The excess liquid coating including that which is wiped from the boards by wiper l8 drain by gravity to splash plate 25 and through the filter trays 64, 52 and 53 to remove gross solid contaminants. The filter liquid coating is collected for recirculation in the reservoir in the base of tank 10.

Having thus described the invention, what is claimed 1. An apparatus for applying liquid protective coatings at less than atmospheric pressure on elongated workpieces having at least three laterally disposed surfaces, said apparatus comprising:

an enclosure having operative means'for maintaining subambient pressure within said enclosure;

said enclosure having an inlet aperture and an outlet aperture located substantially horizontally in-line and defining therebetween a coating application ,zone within said enclosure;

' I said inlet opening having resilient means secured peripherally thereto and adapted to resiliently engage the lateral surfaces of each workpiece for maintain ing a closed enclosure while the workpiece passes through said enclosure;

said outlet opening having peripherally secured thereto a plurality of flexible, resilient applicator means conforming substantially to the periphery of the lateral surfaces of the workpiece and adapted to resiliently wipe all of the laterally disposed surfaces of said workpiece;

a flow-coating dispenser supported within said housing above said coating application zone and operative to flow-coat liquid coating onto workpieces passing through the coating application zone;

a collecting reservoir supported within said housing below said coating application zone for collecting excess liquid coating dispensed by said flowcoating dispenser;

a recirculating pump operatively interconnected to said collecting reservoir and said flow-coating dispenser for recirculating excess coating to said flowcoating dispenser; and

a separator means secured within said housing below said coating application zone, but vertically above the lowermost portion of said reservoir, and adapted for separating solid contaminates from the excess coating material to be recirculated.

2. The coating apparatus of claim 1 wherein the separator means comprises at least two vertically spaced filter trays each having a plurality of openings therein.

3. The coating apparatus of claim 2 wherein the uppermost filter tray has a plurality of 55-inch openings, and each succeeding tray therebelow has openings reduced in size by about one-half relative the openings in the tray located vertically thereabove.

4. The apparatus of claim 1 wherein said flexible wipers comprise an assembly of a plurality of brush-like elements positioned to define an aperture conforming substantially to the periphery of said feed.

5. The apparatus of claim 1 wherein said brush-lik elements are adhesively joined together. 

1. An apparatus for applying liquid protective coatings at less than atmospheric pressure on elongated workpieces having at least three laterally disposed surfaces, said apparatus comprising: an enclosure having operative means for maintaining subambient pressure within said enclosure; said enclosure having an inlet aperture and an outlet aperture located substantially horizontally in-line and defining therebetween a coating application zone within said enclosure; said inlet opening having resilient means secured peripherally thereto and adapted to resiliently engage the lateral surfaces of each workpiece for maintaining a closed enclosure while the workpiece passes through said enclosure; said outlet opening having peripherally secured thereto a plurality of flexible, resilient applicator means conforming substantially to the periphery of the lateral surfaces of the workpiece and adapted to resiliently wipe all of the laterally disposed surfaces of said workpiece; a flow-coating dispenser supported within said housing above said coating application zone and operative to flow-coat liquid coating onto workpieces passing through the coating application zone; a collecting reservoir supported within said housing below said coating application zone for collecting excess liquid coating dispensed by said flow-coating dispenser; a recirculating pump operatively interconnected to said collecting reservoir and said flow-coating dispenser for recirculating excess coating to said flow-coating dispenser; and a separator means secured within said housing below said coating application zone, but vertically above the lowermost portion of said reservoir, and adapted for separating solid contaminates from the excess coating material to be recirculated.
 2. The coating apparatus of claim 1 wherein the separator means comprises at least two vertically spaced filter trays each having a plurality of openings therein.
 3. The coating apparatus of claim 2 wherein the uppermost filter tray has a plurality of 1/2 -inch openings, and each succeeding tray therebelow has openings reduced in size by about one-half relative the openings in the tray located vertically thereabove.
 4. The apparatus of claim 1 wherein said flexible wipers comprise an assembly of a plurality of brush-like elements positioned to define an aperture conforming substantially to the periphery of said feed.
 5. The apparatus of claim 1 wherein said brush-like elements are adhesively joined together. 